Dr_karima_oman.pdf

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Thermal Effects of Converging or Diverging Risers on Solar Hot Water System Asst. Prof. Dr. Karima E. Amori*, Nooraddin Saad Jabouri

Abstract— This experimental study includes two parts; first is a comparison between the performance of two locally fabricated similar solar water heaters. One of the flat plate collectors is a new design of accelerated absorber, its risers are of converging ducts (the exit area is half the entrance). The other collector is of conventional absorber (its risers have the same cross sectional area along its length). The second part is reversing the new collector such that its risers are of diverging type. Each collector is the major part of an indirect thermosyphon circulation solar hot water system facing the south with a fixed tilt angle of 45o from the horizontal. Concurrently experiments are conducted on the two solar water heaters from January to June of 2009 for two different water withdrawal profiles, interrupted and no load. Results show that a considerable enhancement of thermal performance approximately (31.5%) of absorbed heat (useful gain) at solar noon is obtained for the converging risers compared with the conventional type. The maximum instantaneous efficiency reached for the accelerated absorbed flat plate was 60%, while that of conventional absorber was (32%) for the same conditions. The longitudinal water temperature variations in the risers of accelerated absorber were larger than that belong to the conventional absorber. Also the circulation rate in the accelerated absorber was higher when it is compared with the conventional, which in turn indicate higher values compared with absorber of diverging risers. Index Terms— Flat plate collector,Risers,Solar,, Storage tank, stratification. water withdrawal pattern. It is worthy to mention that to carry out a fair comparison, the collectors are made as I. INTRODUCTION identical as possible. OLAR hot water systems function as heat exchangers.

S

They receive solar radiant energy and transfer it to the flowing fluid. The performance of solar systems varies as the design variables change, so it is necessary therefore to predict the parameters affecting this design and the operational variables. Keltt et.al.(1984) [1] studied experimentally the thermal performance of submerged coil heat exchangers for single wall coil and double wall coil for different tank sizes namely (300L and 450 L) for different load flow rates. Khalifa (1999) [2] investigated a thermosyphon domestic hot water system to show the important variables that affect the performance of the solar system such as the temperature variation along the absorber fins, tubes and in the flow direction as well as the thermosyphonic mass flow rate. The design of an efficient storage tank heat exchanger has been investigated by Shokouhmand et.. al. (2008) [3] with different coil pitches and curvature ratios. An enhancement in heat transfer rate is obtained due to the centrifugal force due to the curvature of the tube, results in the secondary flow development. The objective of the present work is to investigate concurrently the thermal performance of a locally made new solar flat plate collector of converging or diverging risers as part of thermosyphon closed type solar water heater compared with (flat plate collector of straight risers) the conventional type hot water system for different

II. THEORY Thermal analysis is covered in many solar thermal engineering texts (Duffi & Beckman [4], and Lunde [5] ). Therefore, only equations which describe the thermal performance of the system will be described in this paper. For collector the hourly useful energy gain can be calculated by:

Q coll = m •coll C p (Tout − Tin )

(1)

The heat obtained by the water withdrawal can be calculated as

Q sup = m •sup C p (T18 − T20 )

(2)

m • mass flow rate (kg/s) , C p water specific heat (J/kg.K), and Ti

Where Q coll heat transferred in W,

measured temperature at location i (oC) . The useful energy enhancement is calculated as:

%enhancemen t=

(usefulenergy )acc. − (usefulenergy )conv. (usefulenergy )conv.

(3)

Manuscript received October 9, 2009. Karima Esmail Amori , Univ. of Baghdad, Eng. College, Mech. Eng. Dept. , Baghdad/ Iraq, ([email protected]) Nooraddin Saad Jabouri Graduated MSc. student

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The collector instantaneous efficiency can be determined according to:

η coll = Q coll / (A coll * IG)

(4)

The instantaneous stored energy in the storage tank is evaluated as

Qstored =

m tan k C p Δt

(Tav 2 − Tav1 )

(5)

Where Tav is water average temperature of the internal

Collector cross section

Accelerated riser

41

cylinder; which is calculated as

Tav = ∑ Ti / 15 i = 27

2,1 refer to end and initial of the time period Δt (taken as half an hour). III. EXPERIMENTAL SETUP Two identical solar water heater are manufactured in this work; one is supplied with flat plate collectors of accelerated absorber, the other is supplied with flat plate collector of conventional absorber. Fig.(1) shows the configuration of solar water heater tested in this work when its storage tank is is of two concentric cylinders.

To load

From main supply

Fig.(1): Close Loop Themosyphon Solar Water Heater A Accelerated Flat Plate Solar Collector A flat plate collector with accelerator plate consist of nine risers equal spaced parallel copper converging rectangular cross sectional area (Fig. 2) of inlet and outlet hydraulic diameter (20mm, 13.3mm) respectively, so the inlet and outlet dimensions are (20x20) mm (10x20) mm respectively , riser length of (120mm), the center to center distance is (100mm), each riser has an equal right and left fin length of (40mm), these risers are connected with two headers of rectangular cross sectional area, one at each end, to collect or distribute water from and into risers, the joints between the headers and risers ends are welded by using brass alloy . A black mat painted copper sheet is used as an absorbing plate of (1.20 x 0.9) m and of (1.9 mm) thickness, this sheet is welded to the risers by brass alloy, the structure are good insulated with a fire wool coating of (70) mm thickness.

Conventional rise Fig.(2): Configuration of manufactured flat plate collector B Conventional Flat Plate Solar Collector Conventional flat plate collector without accelerator plate is designed identical to accelerated flat plate collector, but its riser cross sectional area is of constant hydraulic diameter of 20mm so the inlet and outlet cross section dimensions are (20x20) mm, Collectors box frame are manufactured from (2)mm thickness galvanized sheet formed as a box, the collectors are connected to its containers by screws , the collectors are well insulated by using glass wool insulator of (70) mm thickness from the back of the collectors and (50) mm thickness from the perimeter of the collectors. A glass panel of 4mm thickness is used as a transparent cover of the collector with area of (1.25x0.95) m², the glass cover is lying on the collector frame in a small channel of 25mm wide, it is fixed by using a black silicon. A rubber tape is fixed between the glass and channel to prevent air leakages. The cover-absorbing plate spacing was (70) mm. C Shell and Tube Storage Tank Two concentric horizontal cylinders form the storage tank made from galvanized plate of 2mm thickness, the internal cylinder of inner diameter of (375) mm and (1m) length and capacity of 110L, the outer cylinder of internal diameter (470) mm and length of (1.2m), to form an annular space of 90L, the internal cylinder is used for the close loop water (energy carrier) while the annular space between internal and external cylinders is used for loading water (domestic use water). Each cylinder has two holes to form inlet and outlet ports. The tank is well insulated by a glass wool insulator of (70) mm thickness, the storage tank can be oriented horizontally or vertically as required D Temperature Measurement . (105) calibrated thermocouples (Type T Copper and Constantan) are used to measure the temperature at various points of water tubes, water storage tanks, inlet and outlet of collectors, inlet and outlet of the withdrawal water, ambient and glass cover, as shown in Fig.(3). All these kinds of thermocouples are joined to Digital thermometer reader (Autonics-T4WM/ K(CA) 0-1200).The ambient

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temperature is measured by a using a mercury in glassthermometer

F Test Procedure The thermosyphon circulation solar water flat plate collectors were connected as a closed loop (indirect) system. The experiments were carried out from January to July 2009. Before each test some preparations are made, the closed collector loop was filled with water, the glass cover is cleaned thoroughly and the measurements and apparatus were checked. Then the storage tanks were filled with water and taking the readings each half an hour from sunrise to sunset. The new designed system was tested side by side with the conventional type. The test is conducted from sunrise to sunset for two different load water withdrawals profiles: no load and profile (II) shown in Fig.(4) which is equivalent to daily consumption of single storage tank volume. All temperature measurements, load flow rate, circulation rate and wind speed are recorded.

E Flow Rate Measurement: The mass flow rate of the thermosyphon circulation flat plate collectors is so small, so only the laser flow meter can detect the flow which in not available, so a transparent tube of collector outlet

13 12

7

9

5

6

14

20

L/hr

4

8

3

15 10 5 0 8:00

1

2

3

collector 11 inlet

V

10

flat plate collector

Fig.(3): Coding and Locations of Temperature Measurements 250mm length and 10mm diameter is connected between collector outlet and storage tank inlet. An ink injection is used to calculate the circulation rate by injecting ink and measuring the time required to pass the (250 mm) distance using stopwatch to compute fluid velocity. The water load flow rate is measured by using flow meter of (2-22) L/min.

9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 Local time

Fig.(4): Water Withdrawal Profile II IV. RESULTS AND DISCUSSION Fig(5) shows the measured (each 5 minutes) ambient temperature and solar radiation for the (24) hours of 27-12009 on the solar collector which is obtained from the ministry of science and technology solar center in Baghdad. It is clear that the peak solar radiation was between hours (11 to 13) the peak ambient temperature was (19°C). Fig.(6) a & b show a comparison of the circulation rate between the converging-conventional and divergingconventional respectively flat plate for load profile II. A considerable increase in circulation rate is recorded for the whole test time in case of converging risers, while it is retarded for diverging risers. Since the accelerated riser occupies lesser amount of water at the any cross section especially at riser upper part compared with the straight riser, so the temperature within each riser was higher as shown in fig.(7) which leads to higher decrease in water density then faster circulation. Fig.(8 a.) shows a Good enhancement in the instantaneous collector efficiency for converging risers 60% while reaches 33% for conventional type at solar noon. It has been found that The diverging of risers has unconsiderable effect on the collector efficiency as shown in (Fig8 b.). A little variance in useful heat gain is noted at the early day hours between the diverged and conventional risers as shown in fig.(9), larger deviations

> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < indicated after solar noon since the solar radiation is increased till 2:00PM. Fig(10) shows that the percentage enhancement of absorbed heat for flat plate of converging risers increases till the solar noon (approaches 40%) then it decreases toward the sunset, while a continuous decrease is indicated for diverging risers. Fig.(11) shows the hourly stored heat in the solar hot water storage tank for both converging and conventional risers collectors for no load case. At almost day hours the converging risers improved the stored energy as presented in table (1). While at the last three hours the storage tank average temperature was larger for converging risers as shown in (Fig. 12).

4

0.26

converging riser conventional

4-3-2009

circulation rate (L/hr)

0.24

0.22

0.20

IV. CONCLUSION The following concluding remarks can be drawn during this work which are: 1- Water temperature in the risers increases obviously with the longitudinal direction while no significant variation recorded between risers along collector width near lower header 2- The natural circulation rate in the system is significantly affected by risers type. Converging risers increasing the circulation rate while the diverging risers decreasing it. 3- The instantaneous efficiency of the solar collector is greatly depends on circulation rate and also depends on the temperature difference between collector outlet and inlet temperatures ,so it is affected by type of risers. 4- A considerable thermal performance enhancement is obtained for the new designed converging risers flat plate collector 5- Improvement in stored heat is obtained when using accelerator collector.

0.18

7

0.28

8

200

6 4

100 27-1-2009

2

0

0 0

2

4

6

8

10

12

14

16

18

20

22

time

Fig.(5): Solar Radiation and Ambient Temperature (27/1/2009 at 33.3o N Baghdad)

24

14

15

16

diverg. riser

4_6_2009

circulation rate (L/hr)

0.22

0.20

0.18

0.16

0.14 8

9

10

11

12

13

14

15

16

local time (hr)

(b) Fig.(6): Circulation Rate in the Collector for profile II a). Conven.-converged b). Conven.-Diverged Temp ( C)

Radiation (W/m2)

10

13

0.24

14 12

12

0.26

16

300

11

conventional

ambient temp.

400

10

(a)

18

solar radiation

9

local time (hr)

20

500

8

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45

T3

T3 T6

40

T6

40

T9

T9

35

30

Temp. ( C)

Temp. ( C)

35

a). 4-3-2009

25

25

20

20

15

15

A

10 7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

b). 4-6-2009

30

10 15.0

16.0

7.0

8.0

9.0

10.0

local time (hr)

11.0

12.0

13.0

14.0

15.0

16.0

local time (hr)

converging Riser C-C

conventional Riser C-C

Fig.(7): Sample of Water Temperature distribution along risers 70

60 55

4-3-2009

60

conventional

50

converging riser

45

diverged riser conventional riser

50

efficiency %

effeciency %

40 40

30

35 30 25 20

20

15 10

10

5 0

0 8

9

10

11

12

13

local time (hr)

14

15

16

8

9

10

11

12

13

local time (hr)

Fig.(8): Collector Instantaneous Efficiency For Load profile II of a). Converged- Conventional risers b). Diverged- Conventional risers

14

15

16

5

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6

7

40 6

30

20

enhancement %

useful heat gain W

5

4

3

10

0

2

-10 1

diverging rises

-20

diverging-conventional

conventional risers

converging-conventional

0

-30 11

12

13

14

15

16

8

local time (hr)

9

10

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13

converging risers conventional

900 800 700 600

Heat (W)

500 400 300 200 100 0 -100 -200 -300 -400 7

8

9

10

11

12

13

14

15

16

local time hr

Fig.(11): Instantaneous stored heat in the internal cylinder of the storage tank for no load case 35 30 25 20

conventional

15

converged riser

10

10-2-2009

5 16:00

14:00

13:30

13:00

12:30

12:00

11:30

11:00

10:30

9:30

10:00

9:00

8:30

0 8:00

Temp C

15

16

Fig.(10): Percentage Enhancement In Absorbed Heat Converging or Diverging Risers Compared With Conventional Type

1100 1000

14

local time (hr)

Fig.(9): Collector Useful Heat Gain For Conventional And Diverging Risers

15:30

10

15:00

9

14:30

8

local time hr

Fig.(12)Water Average Temp. of Inlet Cylinder of The Storage Tank for no load case

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with converging risers Temp. oC Heat W 12.27 12.53 68.11 13.53 255.44 15.53 510.88 18.73 817.41 19.6 221.39 20.2 153.26 21.06 221.38 21.93 221.37 21.73 -51.08 23.46 442.77 27.46 1021.77 29.06 408.71 30.73 425.73 32.2 374.65 32.46 68.119 32.73 68.11

with conventional risers Temp. oC Heat W 12.73 12.73 0 13.93 306.53 15.13 306.53 17 476.83 19.26 579.01 20.66 357.62 22. 6. 408.71 23.6 340.59 24.86 323.64 26.26 357.54 26.06 -51.09 26.66 153.27 26.66 0 26.33 -85.16 25.93 -102.17 24.53 -357.6226

REFERENCES [1]

Klett D.E.,Goswami D.Y.,and Saad M.T.,(1984 Aug.)"Thermal Performance Of Submerged Coil Heat Exchangers Used In Solar

[2]

[3]

[4] [5] [6]

[7]

[8]

[9]

7

Energy Storage Tanks", Journal of Solar Energy Engineering,., vol. 106, PP. 373-375 Khalifa A. N., (1999)"Thermal Performance Of Locally Made Flat Plate Solar Collectors Used As Part of a Domestic Hot Water System", Energy Conversion & Management 40, PP. 1825-1833. Shokouhmand H., Salimpour M.R. and Akhavan-Behabadi M.A.,(2008), "Experimental Investigation Of Shell And Coiled Tube Heat Exchangers Using Wilson Plots", International Communications In Heat And Mass Transfer, 35, PP. 84-92. Duffi J. A., and Beckman W. A.,(1974), "Solar Energy Thermal Process ", John Wiley & sons Inc. New York. Lunde P. ,(1980)"Solar Thermal engineering, Space heating and hot Water Systems", John Wiley & sons Inc. New York. Karima E., Amori, Faiz F. Mustafa and Sahar Majeed,(2009)," Solar Water Heater With Shell And Helical Coiled Tube Heat Exchanger As A Storage Tank", accepted for publication in Journal of eng., published by college of eng. ,Baghdad university. Myrna Dayan, (1997),"High Performance In Low Flow Solar Domestic Hot Water Systems", MSc. Thesis Mechanical Engineering University of Wisconsin-Madison. Soteris Kalogirou;(2009);"Thermal performance, economic and environmental life cycle analysis of thermosyphon solar water heaters"; Solar Energy; Vol.83; pp.39-48 Karima E. Amori ; and Nooraddin Saad Jabouri,(2009), "Thermal Improvement Of Solar Hot Water System Using Accelerated Riser In Flat Plate Collector ", The Iraqi Journal For Mechanical And Material Engineering, special issue pp. 244-257.